ICAO Doc 9811 is a document published by the International Civil Aviation Organization (ICAO), which provides guidelines and recommendations for the implementation of Performance-Based Navigation (PBN) in the aviation industry.
The only authorized way to obtain ICAO Doc 9811 is through the ICAO Store . : Approximately $94.00 USD for the digital version. icao doc 9811 pdf link
If you're unable to find the PDF link, you can try contacting ICAO directly or checking with your local aviation authority to see if they have a copy of the document or can provide guidance on how to obtain it. ICAO Doc 9811 PDF Link: A Comprehensive Guide